1. Field of the Invention
The invention pertains to antennas and multiplexers and more particularly to multiplexers for use with antennas and receiving apparatus operating in the AM, FM, CB, weather band (WB), cellular telephone, and/or other specialized frequency ranges.
2. Prior Art
Multiband antennas that simultaneously serve as antennas for AM/FM broadcast radio and for Citizen Band (CB) transceivers are known. A problem in designing antennas of this type is to define an antenna which has near optimal receiving/transmission capabilities in several separate frequency bands. For example, the AM radio band falls in the comparatively low frequency range of 550 to 1700 kHz while FM radio operates in the 88 to 108 MHz range and CB operates in the relatively narrow range of 26.95 to 27.405 MHz. Cellular telephone operates in a frequency band of 825 to 890 MHz. It is well known from antenna design principles that a commonly used electrical length for a rod antenna used with a ground plane is one-quarter of the wavelength of the transmitted signal. Thus, there is a design conflict when a single antenna is used for several frequency ranges. One option used in prior art antenna design is to tune the antenna to the separate frequencies when switching between bands. This has obvious disadvantages to the user of the radio, using impedance matching networks. Another option is to design an antenna which provides a compromise and is usable in several frequency bands. Such an antenna, by its nature, provides near optimal reception in at most one frequency range. For example, it is not uncommon in automobile antennas to use an antenna length equivalent to one-quarter wavelength to the midpoint of the FM range. As a consequence, the lower frequency AM reception is not optimum but is acceptable. However, such an antenna is unacceptable for use with a cellular or CB transceiver. Similarly, a CB antenna does not provide adequate FM or cellular reception.
In automobiles and trucks, it is common to use one antenna for CB and another for AM/FM/WB and a third for cellular telephone and even a fourth for other specialized frequencies. Trucks typically use a pair of CB antennas connected in parallel and through a T-connection to the CB radio equipment. The antennas are often mounted on the side view mirrors on both sides of the cab which, because of their location outside of the cab and beyond the sides of the trailer or box behind the cab, provide a favorable signal reception position. It is not feasible, however, to put separate AM/FM/WB, cellular and CB antennas on the mirrors because of space and interference considerations. Consequently, these antennas have typically been placed in various locations on the vehicle with less than satisfactory signal reception or transmission. For example, reception or transmission for FM and cellular telephone antennas mounted on the roof of a truck cab is often blocked by the box of the truck.
A significant problem in multiple antenna systems of the prior art is the mismatch in electrical characteristics between the two separate antennas of a dual antenna system and the mismatch between the antennas and the radio equipment. Such mismatches result in a loss of power and can cause damage to the radio equipment due to reflected energy. The loss of power is particularly noticeable in fiberglass cabs which lack the standard ground plane.
U.S. Pat. No. 4,229,743 to Vo et al., issued Oct. 21, 1980, discloses a multiband AM/FM/CB antenna having a plurality of resonant frequencies. This prior art antenna uses coil sections wound around portions of the antenna to form a network. The network is used to provide an impedance element having a resonant frequency at approximately 59 MHz. This is an approximate midpoint between the CB and FM band and does not provide optimal reception in the two separate bands.
U.S. Pat. No. 5,057,849 to Dorrie et al., issued Oct. 15, 1991, discloses a rod antenna for multiband television reception. That antenna uses a support rod with two connected windings wound on the rod, one of the windings being spiraled with wide turns and the other being tightly wound. The two windings are capacitively coupled to the antenna connection element by a loop of a third winding. This antenna, when connected to a television receiver, allows the receiver to be switched between UHF and VHF without requiring specific tuning of the antenna. The antenna, however, does not provide optimal reception of two separate frequency bands.
Frequency self-resonant circuits have been used by amateur radio operators to be able to use the same antenna for more than one frequency band. Such known frequency self-resonant circuits customarily consist of a coil in the antenna with a discrete capacitor connected across the coil and external to the coil. Together, the coil and capacitor form an L-C circuit which presents a high impedance at a selected frequency to effectively isolate a portion of the antenna at the selected frequency. Such an arrangement with discrete capacitors is not practical for automotive antennas and other applications.
U.S. Pat. No. 4,404,564 to Wilson, issued Sep. 13, 1983, discloses an omni-directional antenna in which the electrically conductive antenna element is wound around a rod of insulating material and a tuning device comprising a hollow cylinder of non-conductive material mounted on the antenna rod and a metallic sleeve around a portion of the cylinder and an outer coil electrically isolated from the sleeve and the antenna conductor. Such an arrangement does not provide the desired frequency band separation.
U.S. Pat. No. 4,22,053 to Newcomb, discloses an amateur radio antenna constructed of a plurality of telescoping, overlapping tubular sections. The antenna includes a self-resonant circuit comprising a coiled wire section having opposite ends electrically connected to two different telescoping tubular sections which are electrically insulated from each other. The self-resonant circuit has an inductive component provided by the wire coil and a capacitive component provided by the overlapping tubular sections, with the overlapping tubular sections essentially acting as plates of a capacitor. Such overlapping tubular section antennas work well as stationary antennas but are not acceptable for motor vehicle antennas, particularly where relatively long antennas are required, such as for CB transmission and reception. A problem with such prior art multiband antennas is that the antennas are bulky, have too much wind resistance for use on motor vehicles and are not aesthetically pleasing.
Antennas which serve both for cellular telephone and CB are not generally known among commercially available antennas. The difference in operating frequency between the cellular telephone and CB radio is sufficiently great that the designer of a cellular telephone antenna faces an entirely different set of problems than the designer of a CB antenna. The CB antenna operates in a range where a quarter wavelength is approximately 9 feet while the cellular antenna must operate in a frequency range where a quarter wavelength is approximately 3.3 inches. CB antennas are commonly used on trucks and mounted on side mirrors which are spaced apart by approximately 9 feet, or one-quarter wavelength in the CB range to provide and enhance that radiation pattern. Combining a cellular antenna with a CB antenna at that spacing could result in a signal cancellation instead of signal enhancement, depending on the existing ground plane surface. However, a need for a single antenna structure which would serve as an AM/FM/CB/cellular radio antenna has existed for some time. It is recognized that the manufacturer of a single antenna structure is more cost effective both in manufacture and installation and maintenance on the vehicle than a plurality of antennas. Placement and mounting of a plurality of antennas requiring the drilling holes and separate wiring adds to the expense and inconvenience of a proliferation of antennas on a vehicle.
Vehicles such as large trucks typically have a CB transmitter/receiver in addition to an AM/FM/WB receiver, connected to one or more antennae. It is common to add WB frequency coverage to truck and upscale automotive AM/FM automobile radios. This allows a listener to switch the AM/FM/WB radio receiver to weather band frequencies at around 162 MHz to obtain local weather reports. The weather frequencies are relative close to the upper ranges of the FM band that extends to 108 MHz. This allows FM frequency antennas to provide adequate WB reception.
In more recent years, WB frequency range has been added to as a feature to many CB radio sets. In addition, such combination typically includes additional circuitry for detection of alert signals transmitted by weather broadcasting stations in case of severe weather. The alert signal detection circuitry is designed to automatically switch the CB transceiver to the WB broadcast. Since CB and WB both operate within a relatively narrow frequency band, spaced apart from each other, WB reception on CB is typically poor, there is a need for improved WB signal reception on the CB transceiver.
In one prior art arrangement, a weather band frequency trap in the form of a standard coil is added to the CB frequency antenna. However, such a trap adds to the expense of the antenna and, in many prior art antennas, the additional coil tends to weaken the CB antenna performance. Separate antennas are still required to provide AM/FM reception and weather band reception, when weather band reception is received through the AM/FM/WB receiver.
The well-known and widely used cellular telephone system operates within a specified frequency, e.g., 825 to 890 MHz and may use analog or digital transmission between a telephone and a cell site connected to a telephone network. A more recently developed Personal Communication Service (PCS) is a cellular system primarily designed for use in more densely populated urban areas and operates in a higher frequency range of 1850 to 1990 MHz. The standard cellular system is designed primarily for use in relatively open areas and does not function well in congested metropolitan areas where cellular signals may be blocked by buildings and other obstacles. One advantage of PCS is that it operates at a higher frequency with about 10% of the power consumption of a conventional cellular system. Consequently, smaller broadcast antennae are needed, a clear benefit in congested urban areas where conventional cellular towers are impractical. There is also less of a problem with polarization. With aggressive marketing, the demand for PCS capability is increasing.
Dual-band cellular phones are now available that operate at either standard or PCS frequencies and are designed to automatically switch from standard cellular to PCS frequencies when the PCS signal is stronger.
A disadvantage of hand-held portable cellular phones is that they are provided with a relatively small antenna and do not operate optimally when used in a metallic enclosure, such as a typical car or truck. Many vehicles, particularly long-haul trucks, are equipped with a cellular antenna to which a cellular phone may be connected for communication at the standard cellular frequency. A problem with such installations is that signals at the higher PCS frequencies cannot be adequately received or transmitted via the standard cellular antenna. As a result, a person using a cellular phone connected to such a cell phone antenna and entering a congested metropolitan area may lose the standard cellular signal. In order to pick up a PCS signal in the area, the user must physically disconnect the telephone from the cell phone antenna and resort to the antenna on the hand-held portable cellular phone to receive and transmit PCS signals. However, in a metallic enclosure such as a car, the PCS signals are often not optimally received by the portable phone antenna.
Another specialized communication system that is becoming increasingly popular in the commercial trucking industry is provided by Orbcomm L.P. through its Orbcomm(copyright) system. The Orbcomm(copyright) system uses 28 low-earth-orbit satellites to track mobile equipment such as trucks and trailers, as a well as other earth-bound mobile assets. Orbcomm(copyright) transmits over the 148-150 MHz band and receives at 137-138 MHz, both ranges being higher than AM/FM but lower than WB. There remains a need to minimize the number of antennas and the tuning requirements of a mobile antenna while maximizing and distributing the number of frequencies that can be received from the antenna through a multiplexer.
These and other problems of the prior art are overcome by multiplex circuitry that includes a first connection terminal for connection to a first antenna. The first antenna is arranged to receive and transmit signals in a first frequency range. A second connection terminal is provided for connection to a second antenna. The second antenna is arranged to receive and transmit signals in a second frequency range, discrete from and higher than the first frequency range. Preferably the first and second antennas are separate physical antennas, although they can be merely electrically separate on the same physical apparatus.
The multiplexer has a third connection terminal for connection to a signal processor adapted to receive the first frequency range, and a fourth connection terminal for connection to a signal processor adapted to receive the second frequency range. A first filter is connected between the first connection terminal and the fourth connection terminal to exclude signals outside of the first frequency range, and a second filter is connected between the second connection terminal and the third connection terminal to exclude signals outside the second frequency range.
Preferably, the first and second filter circuits each include a capacitor and an inductor in series. Also, the second filter circuit can have another capacitor in parallel with the inductor.
The invention thus provides for the reception and/or transmission of AM, FM, CB, WB, standard cellular telephone signals, PCS telephone signals, and Orbcomm(copyright), while enabling switching between choice of frequencies without any substantial losso degradation of signal. In one particular embodiment of the invention, a multiplexer is provided having a first connection to AM/FM/WB, a second connection to CB, a third connection to standard cellular frequencies, and fourth connection to Orbcomm. Advantageously, no switching or changes in connections are required on the part of the person switching between the respective frequencies.